Abstract

Abstract SS and PP precursors are currently the only body wave data types that have significant coverage in both oceanic and continental
regions to study the existence and characteristics of mantle discontinuities on a global scale. Here, the techniques used
by global seismologists to observe SS and PP precursors are reviewed. Seismograms, aligned on SS or PP, are stacked using
normal move out (NMO) techniques to obtain common depth point gathers. Bootstrap methods are employed to determine 95% confidence
levels of the stacks and robustness of the observations. With these relatively simple techniques, a range of discontinuities
has been found in the mantle up to 1,200 km depth. The stacks are dominated by the transition zone discontinuities at 410,
520 and 660 km depth, but additional discontinuities at 220, 300–350, 800–900 and 1,100–1,200 km depth are also seen in certain
regions. An overview is given of the most recent observational results with a discussion of their mineral physical interpretation
and geodynamical significance. Both seismology and mineral physics agree on the level of complexity at the transition discontinuities:
a simple 410, a more complicated 520 and a highly complicated 660-km discontinuity are consistently found in both disciplines.